JPH0292106A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To optimize a transient response characteristic by switching a holding capacitor with a switch at the time of controlling a gain and at the time of holding a gain. CONSTITUTION:A comparator 12 always compares a rectified signal 8 and a reference voltage, a constant current circuit 13 is on-off-controlled, a holding capacitor CH 2 is charged and discharged and an action is always executed so as to become the same voltage as a holding capacitor CH 1. At the time of holding a gain, when a gain holding signal is a high level, a switching circuit 14 selects the holding capacitor CH 2, the loop of an AGC circuit is shut off and the gain of a voltage control amplifier 1 is determined by a control voltage 11 charged to the holding capacity CH 2. Since for the holding capacitor CH 2, the capacity is largely selected, even when the voltage charged to the holding capacitor CH 2 is reduced by a natural leakage, the degree is small, the refunction of the control voltage 11 is not executed and the amplitude change of an output signal 7 is not executed. Thus, the transient response characteristic is optimized.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an automatic gain control circuit (AGC circuit), and more particularly to an AGC circuit having a gain hold function by cutting off the AGO control function.

Prior Art A conventional AGC circuit having this type of gain hold function has a configuration as shown in FIG. That is, the input signal 6 is input to the voltage control type amplifier 1 and is derived as an amplification amount cuff.

This amplification amount cuff is full-wave rectified by a full-wave rectifier circuit 2 and converted into a DC voltage 8, which is compared in level with a reference voltage in a comparator 3.

The constant current circuit 4 is controlled to be turned on or off according to this comparison output 9, and the output current when the constant current circuit 4 is on is passed through the switch circuit 5 to the hold capacitor CH1.
It is designed to be charged.

Then, the gain of the voltage-controlled amplifier 1 is controlled by the charging voltage 11 of this hold capacitor.

The switch circuit 5 is a switch that turns on and off the loop of the AGC circuit using the gain hold signal 10. When the switch circuit 5 is on, the AGC loop is turned on and the AGC function is activated. When the switch circuit 5 is off, the A and G loops are cut off, and the gain of the amplifier 1 is uniquely determined by the hold voltage 11 charged in the hold capacitor CH1.

In the conventional AGC circuit described above, there are two transient response characteristics: one when the gain is controlled so that the output signal 7 is constant, and the other when the gain is held fixed and the output signal changes according to changes in the input signal 6. The transient response characteristic of is determined by the bold capacitor CH1.

When the step-like amplitude shown in FIG. 5 is input as the input signal 6, as the amplitude of the input signal 6 increases, the full-wave rectifier circuit
Since the rectified signal 8 exceeds the reference voltage ref,
Since the comparator 3 turns on the constant current circuit 4 and allows current to flow, the hold capacitor CH1 is charged by this current, and at the same time operates so that the voltage of the control voltage 11 increases and the gain of the voltage control amplifier 1 decreases. . Moreover, when the amplitude of the input signal 6 becomes small, the full-wave rectifier circuit FI! Since the rectified signal 8 of II2 does not exceed the reference voltage V ref,
The comparator 3 keeps the constant current circuit 4 off, and the gain of the voltage controlled amplifier 1 is determined by the discharge of the hold capacitor C. Therefore, hold capacitor CH1
The smaller the capacitance, the shorter the discharge time of the hold capacitor CH1, and the time T A for the output signal 7 to return to its original amplitude.
Since GC is also shortened, it is necessary to reduce the capacitance of the hold capacitor C.

During gain hold in Figure 6, when the bold signal 10 is set to high level, the switch circuit 5 is opened and the gain of the amplifier is determined by the control voltage 11 charged in the hold capacitor CH1, but the hold capacitor C is charged. The current that is being held naturally leaks, the voltage of the control voltage 11 decreases, and the amplitude of the output signal 7 increases, so it is necessary to increase the size of the hold capacitor CH1.

In this way, the conditions of the hold capacitor CHI when the AGC is on, which controls the gain so that the output signal is constant at 7, and when the gain is held are contradictory, so when the gain controller is 1 to roll and when the gain is held There is a drawback that it is not possible to obtain a transient response characteristic in which both the time and the transient response characteristic are optimal.

Purpose of the Invention The present invention has been made to solve the drawbacks of the conventional ones, and its purpose is to optimize the transient response characteristics for both gain control and gain hold. The purpose of this invention is to provide an AGC circuit that can obtain the desired results.

Structure of the Invention According to the present invention, an amplified output is converted to a DC voltage, the DC voltage level is compared with a reference voltage level, and charging of a hold capacitor is controlled according to the comparison result. An automatic gain control circuit configured to control the gain of an amplifier, the circuit comprising: a first capacitor having a large capacity as the hold capacitor;
and a second capacitor having a smaller capacity than the capacitor, and when the automatic gain control function is turned off, the charging output of the first capacitor becomes the gain control voltage of the amplifier. An automatic gain control circuit is obtained.

flag week Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention, and parts equivalent to those in FIG. 4 are indicated by the same symbols. In addition, in this example,
The case of an AGC circuit of a magnetic recording device will be described, but the present invention is not particularly limited to this.

In FIG. 1, only the different parts from FIG. 4 will be described. In addition to comparator 3, a comparator 12 is provided, and this comparator
The constant current circuit 13, also provided separately, is controlled on/off by the output of 2, and this constant current output is supplied via the switch circuit 14 to the hold capacitor CH2, also provided separately, to control charging. .

Hold capacitor C [12 is bold capacitor CH
The capacitance of the amplifier 1 is selected to be larger than that of the capacitor CH2, and during gain hold, the gain of the amplifier 1 is maintained constant by the charging voltage of this large capacitance hold capacitor CH2. During gain control, the hold capacitor CH2 is controlled by the comparator 12 and the constant current circuit 13 so that it becomes the same as the charging voltage of the hold capacitor CH1.

When the amplitude of a read signal from a magnetic medium changes stepwise and is input to the AGC circuit as shown in FIG.
The signal 8 is rectified at 12. The rectified signal 8 is compared with the reference voltage by the comparator 3 and the comparator 12, and the comparator 3 controls on/off the running current circuit 4 whose output amplitude is always constant, and charges the hold capacitor C. By discharging and changing the voltage of the control signal 11, the output signal 7 is controlled to be constant.

As shown in Fig. 2, the transient response characteristics when the amplitude of the input signal 6 changes from large to small are that the hold capacitor CH1 is discharged quickly because the hold capacitor C is small;
The response time Td of the control signal 11 becomes shorter, and the time TAGC for the output signal 7 to return to its original amplitude also becomes shorter.

The comparator 12 also constantly compares the rectified signal 8 and the reference voltage, and controls the constant current circuit 13 on and off to charge and discharge the hold capacitor CH2, so that the voltage is always the same as that of the hold capacitor C old. It's working. During gain hold, when the gain hold signal becomes high level, the switch circuit 14 selects the hold capacitor C112, the AGC circuit loop is cut off, and the gain of the voltage control amplifier 1 becomes the control voltage 11 charged in the hold capacitor C112. Determined by Hold capacitor C
Since the capacitance of H2 is selected to be large, even if the voltage charged in the hold capacitor CH2 decreases due to natural leakage as shown in Figure 3, the degree of the decrease is small and it can be assumed that there is no decrease in the control voltage 11. . Therefore, output signal 7
It can be considered that there is no change in the amplitude of .

As mentioned above, according to the present invention, by using a switch to switch the hold capacitor between gain control and gain hold, it is possible to optimize the transient response characteristics at both times. There is.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a diagram showing operation waveforms of each part when a step amplitude is supplied as an input signal in the circuit of FIG. 1, and FIG. 4 is a diagram showing an example of a conventional AC, C circuit, and FIG.
FIG. 6 is a diagram showing the operating waveforms of each part when a step-like amplitude is supplied as an input signal in the circuit shown in FIG. 4, and FIG. 6 is a diagram showing the operating waveforms of each part in the circuit shown in FIG. Explanation of symbols of main parts 1... Voltage controlled amplifier 2... Full wave rectifier circuit 3.12... Comparator 4.13... Constant current Circuit 14...Switch circuit

Claims (1)

[Claims] (1) Convert the amplified output to a DC voltage, compare this DC voltage level with a reference voltage level, control the charging of the hold capacitor according to the comparison result, and control the gain of the amplifier by the charging voltage of this capacitor. The automatic gain control circuit is configured with a first capacitor having a large capacitance as the hold capacitor and a second capacitor having a smaller capacitance than the first capacitor, and has an automatic gain control function. An automatic gain control circuit characterized in that when the first capacitor is turned off, the charging output of the first capacitor is used as the gain control voltage of the amplifier.